This application claims Paris Convention priority of DE 102 25 531.8 filed Jun. 10, 2002 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a superconducting high-field magnet coil with at least one radially inner and at least one radially outer coil section, wherein at least the radially inner coil section is wound in a solenoid-shaped fashion with an HTS (high temperature superconductor) band conductor, and wherein at least one superconducting connection is provided between the HTS band conductor of the radially inner coil section and the conductor of the radially outer coil section.
A magnet field coil of this type is disclosed in U.S. Pat. No. 5,319,333. A method of providing contact between two band-shaped superconductors is described by A. N. Iyer et al. in Supercond. Sci. Technol. 13 (2000), pages 187-194.
Nuclear magnetic resonance (NMR) measurements are used for structure analysis in solid-state physics and chemistry and also for imaging methods in medical diagnostics for measuring the spatial density distribution of certain atoms, e.g. protons.
To obtain high resolution in an NMR measurement, maximum magnetic field strengths with simultaneous high temporal stability and spatial homogeneity are desired. These magnetic fields are generated by superconducting magnet coil systems. An electric current of up to several hundred amperes flows in the superconductingly short-circuited magnet coils with minimum loss thereby generating temporally stable magnetic fields of a magnitude of several tesla.
Superconductivity occurs only below a maximum temperature (transition temperature Tc), maximum current density (critical current density jc), and a maximum magnetic field strength (critical magnetic field Hc), which all depend on the material. The maximum value of each parameter thereby depends on the instantaneous magnitude of the two other parameters. When a critical value is exceeded, the material is transferred into the normally conducting state.
So-called high-temperature superconductors (HTS) have considerably larger Tc-, jc- and Hc values compared to typical metallic superconductors such as Nb3Sn or NbTi. For this reason, these materials are preferably used for inner coil sections of high-field magnet coils, which are subjected to the maximum magnetic field strengths.
However, the HTS materials have essentially no ductility and low breaking resistance. Therefore, the brittle, ceramic HTS materials must generally be produced in the geometrical shape of use. A band shape is particularly suitable, wherein the HTS material is embedded in a matrix, e.g. of silver. However, the band-shaped HTS can only be curved in the band direction (for example by flatly disposing the band-shaped superconductor on the peripheral surface of a wheel) and with a certain minimum radius. Strong bends (kinks) considerably reduce the current carrying capacity of the band-shaped HTS.
Production of complicated conductor configurations therefore requires that the HTS materials conductingly contact a ductile metallic superconductor Superconducting High-Field Magnet Coil with Superconducting Joints such as e.g. NbTi. Since these materials have relatively small Hc values, they are preferably used in the outer region of coil configurations.
Contact between two superconducting partial sections of a conductor configuration is basically problematic. Solder having a smaller Hc than the remaining superconducting material is usually used to produce a superconducting electric contact. The solder limits applications for the superconductor configuration. Alternatively, the joint must be provided in a region of reduced magnetic field strength. At least the spatial configuration of the materials used in the magnetic field limits the applications of the overall superconductor configuration.
The situation is particularly problematic with HTS materials: Due to the mechanical properties, guiding of the joints out of the high-field region of a radially inner HTS coil section is nearly impossible, and limitation of the magnetic field strength due to solder or to a ductile metallic superconducting material would render use of HTS materials pointless. For these reasons, the full potential of HTS materials in temporally highly stable and highly homogeneous NMR magnets has not been realized up to this point in time.
To produce joints between two band-shaped BSCCO superconductors, according to lyer et al., loc. cit., the matrix material (typically silver) is etched away on one side of each of the band-shaped superconductors being connected, and the exposed superconductor surfaces are pressed together and aged at a high temperature (lap joint). Alternatively, the front ends of exposed superconductor surfaces abut and are then thermo-mechanically treated (butt joint). In both technologies, the travel of the band direction is continued. No solder is used in either type of joints; however, in the region of the joints, jc is reduced to approximately one third of jc in the band-shaped superconductor. Similar joints are disclosed in U.S. Pat. No. 6,133,814.
To be able to utilize the full potential of HTS materials in superconducting magnet coil systems, the inner coil sections, which are formed from HTS materials, must be superconductingly joined to the outer coil sections consisting of ductile metallic superconducting material without having the contact points or the metallic superconducting material substantially limit the performance of the magnet coil system.
It is therefore the object of the present invention to propose a superconducting connection between a brittle HTS superconducting material and another superconducting material, in particular a metallic superconductor, in a high-field magnet coil system which does not considerably limit use of the magnet coil system and, in particular, the performance of the HTS material.
This object is achieved in a surprisingly simple but effective fashion by a superconducting magnet field coil of the above-mentioned type which is characterized in that the superconducting connection comprises a first superconducting joint between two HTS band conductors where the HTS band conductor of the radially inner coil section is connected in a flat, overlapping fashion with at least one further HTS band conductor such that the two HTS band conductors mutually subtend an angle of between 30xc2x0 and 150xc2x0, preferably approximately 90xc2x0, and a second superconducting joint is provided which is electrically connected in series with the first superconducting joint and is disposed geometrically in a region of a considerably smaller magnetic field strength to connect the further HTS band conductor to the conductor of the radially outer coil section, in an electrically conducting fashion.
The direct transition between the HTS band conductor of the inner coil section and a conductor of the outer coil section is replaced by a first high-field-compatible transition between the HTS band conductor of the inner coil section and a further HTS band conductor, which involves a change in direction and is therefore suited to guide a superconducting material path out of the high-field region, and a second transition from the further HTS band conductor to the conductor of the outer coil section, provided in a low field region. The invention provides at least two joints instead of one joint, to utilize the potential of the HTS material of the inner coil section, in particular the high Hc value.
The high-field region has only one transition from HTS to HTS (first joint) whereas the transition from HTS to the conductor of the outer coil section (second joint) is disposed in a region of reduced magnetic field strength, as is the overall conductor material of the outer coil section. The HTS-HTS contact can be fundamentally produced with high Hc and jc, values and the inventive geometry of the first joint permits the required removal of the second joint out of the high-field region.
U.S. Pat. No. 6,133,814, column 5, line 35 to column 8, line 26, mentions selected HTS materials, which can be used in an inventive high-field magnet coil.
A particularly preferred embodiment of the superconducting high-field magnet coil is characterized in that the HTS band conductor of the radially inner coil section and the further HTS band conductor, which are connected at the first superconducting joint, are made from the same HTS material. Disadvantageous selection of different HTS materials at the first superconducting joint could lead to an excessive reduction in jc and/or Hc in this region. This risk is largely minimized by using the same HTS materials in the band conductor of the radially inner coil section and in the further HTS band conductor. In this manner, the characteristics of only one material must be taken into consideration for the manufacture and tailoring of the properties of the first superconducting joint.
In another, particularly preferred embodiment, the further HTS band conductor and the conductor of the radially outer coil section, which are connected in an electrically conducting fashion at the second superconducting joint, are made from different superconducting materials. In particular, the conductor of the radially outer coil section contains metallic superconductors, preferably Nb3Sn or NbTi. This embodiment utilizes the advantages of the invention, since NbTi has good ductility (and is thereby easy to handle) and is used as a standard in the low field region of magnet coil systems, in particular for electric contact with external current sources.
One embodiment of the inventive superconducting high-field magnet coil is preferred with which the superconducting high-field magnet coil is superconductingly short-circuited during operation. This is the standard configuration in NMR operation and exhibits high temporal stability for the magnet field generated by the inventive high-field magnet coil.
In another embodiment, the radially outer coil section projects past the radially inner coil section in an axial direction. This configuration generates a magnetic field with good homogeneity and also corresponds to the standard geometry of magnet coil systems. For this geometry, no previous solution to the object of the present invention had been found.
In a further preferred embodiment, the HTS band conductor of the radially inner coil section is connected to a plurality of substantially parallel further HTS band conductors at the first superconducting joint. The increased number of contact locations increases the surface contact between the two HTS band conductors and thereby the absolute current carrying capacity of the superconducting joints.
In one additional preferred embodiment of the superconducting high-field magnet coil, the HTS band conductor of the radially inner coil section is connected on two opposite sides to further HTS band conductors in a flat overlapping fashion at the first superconducting joint. This increases both the mechanical stability of the contact as well as the contact surface between the two HTS band conductors and therefore the absolute current carrying capacity of the first superconducting joint.
In another preferred embodiment, the angle between the HTS band conductor of the radially inner coil section and the further HTS band conductor is selected such that the further HTS band conductor is guided along the axially outer edges of radially further outward coil sections which axially project past the inner coil section. This configuration saves space and permits compact construction of the high-field magnet coil.
In another embodiment of the inventive superconducting high-field magnet coil, the radially inner coil section is wound on a winding body having end flanges and the first superconducting joint is disposed in a recess of an end flange. This limits the radial extension of the magnet coil system to the inner or outer coil radius and the first superconducting joint does not project into the region of the bore or the region of radially further outward coil sections.
The present invention also concerns a superconducting joint between two HTS band conductors for use in a superconducting high-field magnet coil of the above-described type, with which the two HTS band conductors subtend an angle of between 30xc2x0 and 150xc2x0, preferably approximately 90xc2x0, at the superconducting joint.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.
The invention is shown in the drawing and is explained in more detail with reference to embodiments.